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Wastewater Story – Class 7 Science (Notes & Questions)

Chapter overview: Wastewater Story explains what wastewater is, where it comes from, why it can be harmful to people and the environment, and how it can be treated or safely reused. This chapter links science with everyday life: it shows how our actions affect rivers, lakes, groundwater and why proper management of wastewater is important for health and the environment. The notes below are easy to follow and exam-focused, followed by a full set of practice questions (MCQs, very short, short and long answers) to help you prepare effectively.

Notes — Wastewater Story (Clear & Exam-friendly)

1. What is wastewater?

Wastewater is water that has been used in homes, industries, farms or cities and contains impurities. Examples include water from sinks, bathrooms, toilets, washing machines, kitchens, drains, and industrial effluents. Wastewater contains organic matter, dirt, soap, oils, human waste, chemicals and sometimes disease-causing microbes.

2. Sources and types of wastewater

Domestic wastewater: from households — greywater (from bathing, washing) and blackwater (from toilets). Industrial wastewater: from factories — may contain chemicals and heavy metals. Stormwater/runoff: rainwater that picks up pollutants from streets and roofs. Understanding the source helps decide how to treat wastewater.

3. Why is untreated wastewater a problem?

Untreated wastewater spreads pathogens (causing cholera, typhoid, diarrhoea), pollutes freshwater sources, reduces oxygen in water (leading to fish death), and causes eutrophication — excessive growth of algae due to nutrients like nitrogen and phosphorus. Polluted water is unsafe for drinking, agriculture and harms ecosystems.

4. Components of wastewater

Typical pollutants include suspended solids (mud, sand), dissolved organic matter (food, oils), nutrients (nitrates, phosphates), pathogens (bacteria, viruses), chemicals (detergent residues, pesticides), and heavy metals (lead, mercury) from industrial discharges.

5. Simple measures at home to reduce wastewater impact

  • Use less water: fix leaks, use a bucket for small washing tasks.
  • Avoid pouring oils, paints, medicines, or chemicals down the drain.
  • Segregate waste: compost vegetable waste instead of washing it down the sink.
  • Use biodegradable soaps and detergents when possible.
Classroom-safe activity (suggested): Build a simple model filter using layers of gravel, sand and charcoal in a clean plastic bottle to observe how solids get trapped. Do not use actual sewage for safety — only show the effect using muddy water.

6. How wastewater is treated — basic steps

Treatment involves physical, biological and chemical methods:

  1. Screening & Grit removal: Removes large solids like plastics, rags, and sand.
  2. Primary treatment (sedimentation): Slower particles settle down in tanks.
  3. Secondary (biological) treatment: Microbes break down organic matter in aeration tanks (activated sludge) or trickling filters.
  4. Tertiary treatment & disinfection: Removes nutrients and kills pathogens using chlorination, UV or filtration, making water safe for limited reuse.
  5. Sludge management: Settled solids are treated separately (composted or digested) and converted into manure or used in biogas plants.

7. Reuse and safe disposal

Treated wastewater can be reused for agriculture (irrigation), industrial cooling, or for flushing toilets — reducing freshwater demand. However, reuse must follow safety standards: properly treated water only, and appropriate crops chosen for irrigation.

8. Community & public health perspective

Proper wastewater management reduces waterborne diseases and protects drinking water sources. Sewage systems, community toilets, and treatment plants are public health investments. Awareness and small household actions make a big difference.

Exam tips:
  • Remember the difference between greywater and blackwater.
  • List the treatment steps in order for long-answer questions.
  • Use real-life examples (e.g., septic tank, Sewage Treatment Plant) to score in application-based questions.

Questions & Answers — Practice (MCQs, Very Short, Short & Long)

7 MCQs (choose one correct option)

MCQ 1: Which of the following is an example of blackwater?
A. Rainwater from the terrace   B. Water from washing vegetables   C. Toilet water   D. Water from air-conditioner

Ans: C. Toilet water — blackwater contains human waste and toilet flushing water.

MCQ 2: Which treatment step mainly removes suspended solids by settling?
A. Screening   B. Sedimentation (primary treatment)   C. Chlorination   D. Filtration

Ans: B. Sedimentation (primary treatment).

MCQ 3: Eutrophication is primarily caused by excessive amounts of:
A. Dissolved oxygen   B. Nutrients (nitrogen & phosphorus)   C. Heavy metals   D. Sand

Ans: B. Nutrients (nitrogen & phosphorus).

MCQ 4: Which of the following is NOT recommended to pour down the drain?
A. Vegetable peels   B. Used engine oil   C. Soapy water   D. Clean water

Ans: B. Used engine oil — it contaminates water and is hard to treat.

MCQ 5: Which method uses microorganisms to break down organic matter?
A. Chlorination   B. Activated sludge (biological treatment)   C. Grit removal   D. Sedimentation

Ans: B. Activated sludge (biological treatment).

MCQ 6: Which of these is a safe reuse of treated wastewater?
A. Drinking directly   B. Irrigation of non-edible crops   C. Infant formula   D. Bottled water production

Ans: B. Irrigation of non-edible crops — reuse must be matched to the level of treatment.

MCQ 7: What is the main aim of disinfection in wastewater treatment?
A. Remove oils   B. Kill or reduce pathogens   C. Remove sand   D. Increase nutrient content

Ans: B. Kill or reduce pathogens (e.g., by chlorination or UV).

7 Very Short Answer Questions

  1. Q: Define greywater.
    A: Greywater is wastewater from baths, sinks and washing machines that does not include toilet waste.
  2. Q: What is screening?
    A: Screening removes large floating objects and debris from incoming wastewater using grates.
  3. Q: Give one example of industrial wastewater.
    A: Effluent from a dyeing factory that contains coloured chemicals.
  4. Q: What does 'sludge' mean?
    A: Sludge is the solid material that settles out during treatment and is further processed.
  5. Q: Name one disinfectant used in treatment.
    A: Chlorine (chlorination).
  6. Q: Why avoid pouring oil down the drain?
    A: Oil floats and creates pollution that is hard to remove in treatment plants.
  7. Q: What is septic tank used for?
    A: On-site treatment of household sewage in areas without a centralized sewer system.

7 Short Answer Questions

  1. Q: Explain the difference between domestic wastewater and industrial wastewater.
    A: Domestic wastewater comes from homes and contains soaps, food residues and human waste. Industrial wastewater often contains chemicals, heavy metals or high biological load, and needs special treatment before discharge.
  2. Q: What is biological treatment? Give one example.
    A: Biological treatment uses microbes to decompose organic matter. Example: activated sludge process used in many treatment plants.
  3. Q: Mention two harmful effects of untreated wastewater on aquatic life.
    A: Reduced oxygen levels (fish die), and algal blooms (eutrophication) that change ecosystems.
  4. Q: What role do households play in reducing wastewater pollution?
    A: Households can reduce water use, avoid disposing hazardous materials down the drain, and segregate food waste for composting.
  5. Q: How is sludge typically handled after removal?
    A: It can be digested anaerobically to produce biogas, composted into manure, or dried and used as soil conditioner after safe processing.
  6. Q: Why is nutrient removal important in wastewater treatment?
    A: To prevent eutrophication and maintain water quality for aquatic life and human uses.
  7. Q: What is decentralised wastewater treatment?
    A: Small-scale treatment systems close to the source (e.g., septic tanks, constructed wetlands) for villages or buildings not connected to city sewers.

7 Long Answer Questions

  1. Q: Describe the main stages of a typical sewage treatment plant and explain the purpose of each stage.
    A: Treatment starts with screening to remove large objects. Grit removal removes sand and heavy particles. Primary sedimentation allows solids to settle as sludge. Secondary (biological) treatment uses microbes to decompose dissolved organic matter. Secondary sedimentation separates the microbial biomass. Tertiary treatment removes nutrients and fine particles. Finally, disinfection (e.g., chlorination or UV) reduces pathogens. Sludge is treated separately for safe disposal or reuse.
  2. Q: Explain how untreated wastewater can affect human health and give two preventive measures communities can adopt.
    A: Untreated wastewater often contains pathogens that cause diarrhoea, cholera and typhoid. It can contaminate drinking water and food. Preventive measures: ensure proper sewage disposal and treatment, provide clean drinking water, and promote handwashing and sanitation practices. Community toilets and safe sewage systems reduce disease spread.
  3. Q: Discuss the environmental impacts of nutrient-rich wastewater entering lakes and rivers.
    A: Nutrients like nitrates and phosphates promote rapid growth of algae and aquatic plants. When these die and decompose, microbial activity consumes dissolved oxygen, creating low-oxygen zones (dead zones) where fish and other organisms cannot survive. This disrupts food chains and biodiversity. Controlling nutrient discharge through treatment and better agricultural practices helps mitigate eutrophication.
  4. Q: How can treated wastewater be safely reused in agriculture? Explain conditions and precautions.
    A: For reuse, water must be treated to remove pathogens and harmful chemicals. Treated water suitable for irrigation should meet safety standards and be used on crops that are not eaten raw, or applied by drip irrigation to reduce contact. Regular monitoring and proper crop selection, along with farmer training, are essential to ensure food safety.
  5. Q: Compare centralized and decentralized wastewater treatment approaches with examples.
    A: Centralized systems collect sewage via large sewers to a big treatment plant (common in cities). They can treat large volumes efficiently but need infrastructure and investment. Decentralized systems (septic tanks, constructed wetlands) treat wastewater near the source; they are suitable for rural areas or small communities and are flexible and cost-effective for areas without sewer networks.
  6. Q: Describe a safe, classroom-friendly experiment to show filtration and explain the science behind it.
    A: Create a layered filter in a plastic bottle: cotton at the bottom, then sand, fine gravel, and coarse gravel. Pour muddy water on top and observe clearer water collected at the bottom — mechanical filtration removes larger particles. Explain that real treatment uses similar steps but also biological and chemical methods to remove dissolved pollutants.
  7. Q: Why must industries treat wastewater before discharge? Provide two examples of industrial pollutants and their effects.
    A: Industrial wastewater can contain harmful chemicals and heavy metals that persist in the environment and bioaccumulate in food chains. Examples: effluent from textile industries with dyes can reduce light penetration in water, affecting photosynthesis; metal plating industries may release chromium or lead, which are toxic to humans and animals. Treating such effluents prevents environmental damage and health hazards.

Final note: Wastewater management is a mix of science and community action. By understanding how wastewater behaves and how treatment works, students can become active citizens — reducing pollution at the source and supporting solutions that protect health and the environment.

Keep learning, act responsibly, and share these ideas with your family — small habits create big change.